1) Field of the Invention
The present invention relates to a thermoplastic resin composition and laminates thereof, and more particularly to a thermoplastic resin composition useful as a raw material for moldings having distinguished characteristics such as mechanical strength, impact strength, elasticity, gas barrier property, adhesiveness, antistaticbility, low temperature heat sealability, transparency, coatability, miscibility, moldability, radiation resistance, etc., and to laminates thereof.
2) Prior Art
Thermoplastics have been widely used in fields of industrial materials, structural materials, medical instrument materials, packaging materials for food, medicines, cosmetics, industrial articles, etc., laminated steel plates, automobile materials, etc., on the basis of their characteristics. However, they still have problems due to recent increasing demands for higher quality.
Polyolefins have been used in various fields owing to their low cost, high mechanical strength, good sanitation, and good moldability. However, polyolefins have poor oil resistance and gas barrier property, and when they are used as containers for food such as mayonnaise, soy sauce, etc., it is impossible to preserve these kinds of food for a long time. When they are used as gasoline containers, there are such drawbacks as a large amount of gasoline leakage and swelling of containers leading to container deformation. To overcome the drawbacks, it has been so far proposed to laminate the polyolefins with polyester (which may be hereinafter referred to as PET), polyamide (which may be hereinafter referred to as PA), a saponification product of ethylene-vinyl acetate copolymer (which may be hereinafter referred to as EVOH), aluminum foils, glass or the like through an adhesive layer, or to vapor-deposit a gas barrier material of metal, metal oxide or the like onto the polyolefins.
For example, it is known to conduct lamination by applying a dry lamination adhesive such as a polyurethane-based adhesive, a polyacrylic adhesive or a polyester-based adhesive, or to conduct co-extrusion laminating polyolefins graft-polymerized or copolymerized with an unsaturated carboxylic acid or its anhydride as an adhesive resin for lamination. Furthermore, it is known to use olefins graft-polymerized with an acrylic resin derivative or the like as an adhesive resin for the polyolefin and EVOH (JP-A 60-165242), to use a mixture of the graft-polymerized polyolefins with carboxy-modified olefinic polymers or copolymers (JP-A 60-187550), or to use a mixture of olefinic polymers or copolymers with the grafted polyolefins (JP-A 60-184839). However, delamination of the laminates obtained even by these processes has been often observed owing to unsufficient adhesion strength.
Polypropylene is generally cheap and is used in various fields on the basis of its distinguished characteristics such as distinguished transparency, mechanical strength, heat resistance, gloss, chemical resistance, oil resistance, rigidity, flex fatigue resistance, etc. Among polypropylene (which may be hereinafter referred to as PP), homo PP has a high glass transition temperature and thus has such drawbacks as high brittleness at a low temperature, easy breakage, poor transparency, etc. PP has such drawbacks as a poor coatability, a high susceptibility considerable deterioration of physical properties when subjected to irradiation of radiation, or a poor heat sealability at a low temperature. In attempts to overcome these drawbacks, for example, in attempts to improve the transparency, it is known to add a dibenzylidene sorbitol derivative as a nucleating agent to PP and obtain the transparency due to the effect of the nucleating agent, as disclosed in Polymer Digest, July issue (1984), page 116 and Polymer Digest, June issue (1985) page 43. However, these procedures still have such problems as decomposition of the nucleus-making agent during the molding, emission of disagreeable odor, decrease in the impact resistance due to the effect of the nucleus-making agent, etc.
Furthermore, it is known to use special molding procedures, for example, to add hydrogenated terpene resin and a nucleating agent to PP, followed by molding and quenching, thereby to obtain a transparent sheet (JP-A 1-171833 and JP-A 1-171834) or to add an ethylene/.alpha.-olefin copolymer to PP, followed by molding and quenching below 60.degree. C. (JP-A 1-299851). However, these procedures require a special molding machine, because it is difficult to readily obtain a molding of high transparency by ordinary molding machines.
In attempts to improve the impact resistance, it is known to add 5 to 20% of polyisobutene to PP (JP-B 35-10640), to add 5 to 20% of ethylene/propylene copolymer (JP-B 35-7088), to add a lower pressure process polyethylene of high molecular weight to PP (JP-B 37-6975), or to add polyethylene (which may be hereinafter referred to as PE) and ethylene/propylene rubber to PP, thereby making the so called propylene/ethylene block copolymer [Kuroda: Kobunshi (Polymer), Vol. 28, No. 10, page 714 (1979)]. However, these procedures still have such problems as poor luster and poor transparency.
Furthermore, it has been proposed to add an ethylene-vinyl ester copolymer and/or ethylene-acrylate copolymer to PP (JP-A-51-145553), or to add a modified specific ethylene-propylene copolymer obtained by graft polymerization with an unsaturated dicarboxylic acid or its anhydride to PP (JP-A 61-89239). These procedures still have such problems as a poor effect on the improvement of coatability and a poor compatibility.
Among polyolefins, PP has an excellent flexing characteristic (hinge characteristic), but the flex crease turns white, or when a tension is applied to PP, the site on which the tension has been applied turns white. That is, a phenomenon of local whitening, so called stress whitening, appears.
To prevent the whitening of PP, it has been so far proposed to add a lubricant or a plasticizer to PP, followed by kneading. However, this procedure has such problems as appearance of bleeding and blooming and deterioration of transparency.
Furthermore, in the field of medical packaging, PP moldings are usually irradiated with gamma ray of cobalt 60 or electron beams as a sterilizing means. However, such a sterilization treatment of PP deteriorates PP, that is, lowers the physical properties such as impact resistance, stretching elongation, etc. To solve the problems, various procedures including modification of PP have been proposed (JP-A 60-215047 and JP-A 61-73711). However, all these procedures have such drawbacks as insufficient transparency, and insufficient effect of control on decreases in physical properties such as impact resistance and stretching elongation after the irradiation with radiation beams.
Furthermore, it has been proposed to improve the heat sealability of PP at a low temperature by adding polybutene-1, or an ethylene/propylene elastomer, or an ethylene/propylene/butene-1 copolymer to PP (JP-A 54-28351 and JP-A 54-603498). However, these procedures still have such a drawback as insufficient heat sealability at a low temperature.
Since polyvinyl alcohol (which may be hereinafter referred to as PVA) has a melting temperature and a heat decomposition temperature very close to each other, it has been impossible to conduct melt-extrusion molding of PVA. The PVA film is soft and tough under a high humidity, but loses the flexibility under a low humidity to become brittle and thus readily breakable. It has been proposed to solve these problems, for example, by adding EVOH to PVA (JP-A 49-33945) or by adding other polymers than polyolefins to modified PVA (JP-A 49-117536) or by other means (JP-A 50-123151). However, all these procedures still have insufficient melt moldability and a poor gas barrier property.
Generally, PA is tough, but an impact resistance as one of indicators of toughness is considerably increased by moisture adsorption. The impact resistance of PA right after the molding or in the dry state during the winter season or at a low temperature is not always satisfactory. That is, cracks are often generated on the PA moldings.
In attempts to improve the impact resistance of PA, it is known to add a copolymer resin of polyolefin and .alpha.,.beta.-unsaturated carboxylic acid or its ester, graft-modified polyolefin resin with an .alpha.,.beta.-unsaturated carboxylic acid represented by maleic anhydride, ionomer copolymer resin of ethylene and methacrylic acid or its ester ionized by Na, Zn, Mg, or the like, or graft-modified ethylene/propylene/diene copolymer rubber with maleic anhydride to PA (JP-A 51-70254, JP-B 44-29262, U.S. Pat. No. 3,845,163 and JP-B 54-44108). However, these procedures still have such problems as (1) poor compatibility with PVA, (2) local generation of gel by an organic peroxide during the graft modification, (3) generation of much gel by reaction with PA, when the polymer contains unreacted monomers, resulting in yellow discoloring, and (4) decrease in the gas barrier property by blending, which follows the addition.
EVOH is used as packaging materials for food, cosmetic or medicines, which require a gas barrier property particularly to oxygen.
However, EVOH is hard, very brittle and readily breakable. Since EVOH has hydroxyl groups in the molecule, it can readily absorb moisture such as water and water vapors, and its gas barrier property is much more deteriorated by moisture absorption. To overcome these drawbacks, EVOH is used as a multilayer laminate by laminating EVOH with one or more thermoplastic resins, such as polyolefins (for example, PE, PP, etc.), PET, polystyrene, polyvinyl chloride, etc.
The lamination procedure includes co-extrusion molding process or extrusion lamination molding process for extruding a thermoplastic resin such as polyolefins, etc., EVOH and resin for bonding these polymers, thereby making a laminate, a dry lamination molding process for individual molding of these polymers, followed by lamination, a solution coating molding process, where the resulting laminate is in the form of sheets, films or bottles.
The laminate sheets are used to serve as container packages, and thus the raw sheets of multilayer laminates are subjected to a secondary heat molding processing including high temperature stretching such as vacuum molding or compressed air molding.
In the secondary processing, the molding conditions mostly depend on the polymer resin having an intermediate melting point among the constituent materials for the multilayer laminates. That is, the secondary heat molding processing is carried out under these molding conditions.
Thus, EVOH, which usually belongs to the material species having a high melting point, is often subjected to the secondary processing under in appropriate molding conditions, and thus fine spaces (so called voids) or cracks are liable to form in the layers. Furthermore, such voids, cracks or local unevenness in the thickness, etc. are liable to form due to higher crystallization rate of EVOH.
The laminate films undergo flexing movements in the pouch-making step, packaging step, distribution steps, etc., resulting in occurrence of pinholes or cracks in the EVOH layer. Thus, the gas barrier property of EVOH is often deteriorated considerably.
In attempts to solve these problems, a method for blending EVOH with PA (JP-A 59-20345), a method for mixing EVOH with glycerin (JP-A 53-88067), a method for blending EVOH with lithium chloride, etc. (JP-A 61-281147), etc. have been proposed. However, the blending of EVOH with PA has a problem that PA reacts with EVOH during the melt molding, resulting in formation of gel or fish-eyes. The blending with a plasticizer such as glycerin, etc. has a problem that the plasticizers have a poor compatibility with EVOH and thus bleeding of these compounds takes place with time, i.e. the plasticizers move towards the boundary surface to the adhesive resin layer and the bonding strength to the layer is lowered with time. Furthermore, the blending of EVOH with lithium chloride, etc. has a problem that the distribution is often poor at the blending, or more cracks or pinholes are generated due to voids formed when the added compounds are subjected to heat stretching treatment.
As described above, pinholes, cracks, unevenness in thickness, etc. are very liable to appear when a multilayered laminate of EVOH is formed or processed into containers, and consequently the gas barrier property is largely deteriorated.
Furthermore, the multilayered laminate has another problem of effective utilization of scraps. For example, when a multilayered laminate film is processed into containers, burrs are formed up to about 40%.
As a process for recovering such scraps, it has been attempted to reuse a regrind obtained by grinding the foregoing scraps. For example, there are suggested a process for providing a regrind layer composed of the regrind on the multilayer laminate or a process for mixing the regrind in the outermost polyolefin layer, etc.
However, since polyolefin and gas barrier resin generally have a poor miscibility between both, and polyolefin and gas barrier resin in the regrind form an unhomogenous mixture, the following troubles during mold-processing at the time of recovery occur. That is, when a regrind layer is provided, adhesion between the regrind layer and gas barrier layer is inferior, so that peeling readily takes place, or the part of gas barrier resin in regrind is readily peeled off as fine fibrous pieces, or foreign matters readily result from uneven interlayer separation. Such foreign matters increase in a prolonged operation, resulting in formation of projections or granular structures on the ultimate products, when observed from the outermost layer side of multilayered laminate, and thus the projections or granular structures considerably deteriorate the appearance of the multilayered laminate, or in the worst case these projections or granular structures break through the resin layers at the outer side or lower the mechanical strength.
In the coextrusion molding, a known adhesive resin is generally used as a miscibility agent for the regrind component. However, such use of the known resin is not satisfactory yet, because of poor miscibility with EVOH or PA, and unreacted monomers, if present in the adhesive resin, readily react with EVOH or PA, resulting in much formation of gels and yellow discoloring as problems.
Multilayered laminates using a hygroscopic gas barrier of EVOH, PA, PVA or the like have such characteristics that permeation of a gas such as oxygen, etc. can be sufficiently suppressed at a low humidity to prevent oxidation of contents in containers, and the quality of the contents can be stabilized for a prolonged time. However, when the multilayered laminates are used in boiling water at 90.degree. to 135.degree. C. as in the form of retort food packages, water permeates into the multilayered laminates from the outermost layer and is absorbed into the gas barrier layer. As a result, the gas permeation resistance of the gas barrier layer is abruptly lowered. That is, the multilayered laminates cannot be used in the pouch food application, etc.
For example, when a multilayered laminate consisting of a PP layer, an adhesive resin layer and an EVOH layer and having a total thickness of about 100 .mu.m is subjected to a retort treatment in boiling water at least at 120.degree. C., water absorption of the EVOH layer amounts to 7% by weight or more, and the oxygen permeation increases to 1,000 times as much as that before the retort treatment in some cases.
In attempts overcome these problems, it is proposed, for example, to add a drying agent comprising an inorganic metal salt to the adhesive layer that binds the gas barrier layer to the outer layer so that the adhesive layer can capture the permeated water, thereby preventing the deterioration of the gas barrier property (European Patent No. 59274, U.S. Pat. Nos. 4,407,897 and 4,464,443; JP-A 57-170748). However, even in this proposed procedure, the interlayer bonding strength is decreased between the adhesive layer and the gas barrier layer, or the use is inevitably limited due to the presence of the inorganic metal salt as foreign matters.
Furthermore, a method for adding a phenol compound to improve the water resistance (U.S. Pat. Nos. 4,347,337 and 4,289,830), and a method for forming the outermost layer of polycarbonate resin having a good water vapor permeability or PA or its mixture in close proximity to the EVOH layer (JP-A 1-253442; JP-A 1-308626; European Patent No. 322891) are known. However, these methods may be effective for thin products such as films, but not for thick product such as sheets, bottles, etc. and they are costly. In case of polycarbonate resin, it is difficult to conduct multilayered extrusion and thus a special extruder is required.
On the other hand, in multilayered laminates containing an EVOH layer, attempts have been made to provide an EVOH layer at the outer side to accelerate restoration of the gas barrier property after the retort treatment [Paper, Film & Foil Converter, October issue, page 54 (1985); Food Package, November issue, page 82 (1984); ibid, December issue, page 67 (1985); COEX '89, page 267 (1989), held by Scotland Business Research, Inc. (USA)]. However, in these attempts, the resulting laminates take an asymmetrical multilayer structure in the thickness direction, and thus requires an additional extruder in the molding, in contrast to the coextrusion molding for producing a symmetrical multilayer structure in the thickness direction, which requires only one extruder for the outer layer resin. Thus, the molding is costly and the restoration of the gas barrier property is not satisfactory yet.
Recently, an attempt has been made to use EVOH having a high content of ethylene units and set the EVOH layer to a large thickness in advance in anticipation of a deterioration of the gas barrier property due to the retort treatment [Future-Pak '90, page 125 (1990), Ryder Associates, Inc. (USA)]. However, the attempt has such a disadvantage as an inevitable high cost.
Still furthermore, a method for blending a polymer reticular structure such as a reticular structure of vinyl alcohol-acrylate copolymer, etc. into any one of the oxygen barrier layer, the outer layer and the adhesive layer (JP-A 62-182030; JP-A 62-208344), and a method for blending an inorganic powder such as mica powder, sericite powder, etc. into the outer layer and the EVOH layer (JP-A 2-47139) have been proposed. However, the former has a problem in the molding due to poor melt moldability and heat stability of the polymer reticular structure, whereas the latter has problems due to the hardness and the resulting brittleness caused by the blending of inorganic powder.